1. Field of the Invention
The present invention relates to such as a wavelength tunable laser with a wavelength monitor (hereinafter called merely “a wavelength tunable laser”) in which the wavelength monitor detects wavelength. The wavelength tunable laser is used for, for example, a WDM (Wavelength Division Multiplexing) transmission system.
2. Description of the Related Art
Embarking on a broadband era, the WDM transmission system which enables communication through a plurality of light wavelengths per system is introduced increasingly in order for an effective use of an optical fiber. Recently, a DWDM device (a Dense Wavelength Division Multiplexing) is also used extensively which multiplexes several dozen of light wavelengths and enables to transmit in higher-speed. Accordingly, a light source corresponding to every light wavelength is required by each WDM transmission system, and a numbers of light sources are required increasingly along with the high-multiplexing. A ROADM (Reconfigurable optical add/drop multiplexers) which adds/drops an optional wavelength at each node is also examined recently for commercial use. If the ROADM system is introduced, a transmission capacity can expand by multiplexing a wavelength and optical paths can be switched by tuning a wavelength, which leads to an optical network with high flexibility.
A DFB-LD (Distributed feedback laser diode) oscillates at a SLM (Single longitudinal mode) is conventionally used widely as a light source for the WDM transmission system because it is easy to use and has high reliability. The DFB-LD has a diffraction grating in about 30 nm depth formed in the whole area thereof, so that a SLM oscillation is stably obtained at a wavelength corresponding to a product of a grating cycle by twice times of an equivalent refractive index. However, because the DFB-LD is unable to tune oscillating wavelengths in a wide range, a WDM transmission system needs manufactures with different wavelengths at every ITU (international telecommunication unit) grids. Therefore, the cost of a shelf control and excess stocks to repair has been increased because different manufactures are required by each wavelength. Moreover, if a normal DFB-LD is used in the ROADM which switches optical paths according to a wavelength, a tunable range of wavelength window is limited in about 3 nm which can be varied by temperature change. Thus, it is difficult to configure an optical network fully leveraging the feature of ROADM which actively utilizes wavelength resources.
In order to overcome the problem with the present DFB-LD and to achieve a SLM oscillation in a wide wavelength range, the intense study is now under way for a wavelength tunable laser. In the followings, a conventional wavelength tunable laser is explained showing some examples from a non-patent literature 1.
The wavelength tunable lasers are classified in two types, one has a wavelength tunable mechanism inside of its laser device and the other has a wavelength tunable mechanism outside of its laser device.
As the former type, a DBR-LD (Distributed Bragg Reflector Laser Diode) is proposed where an active region to generate a gain and a DBR region to reflect light with a diffraction grating are formed in the same laser device. A wavelength tunable range of this DBR-LD is about 10 nm at the widest. A DBR-LD using an uneven pattern of diffraction grating is also proposed as the former type where an active region to generate a gain and a DBR regions placed in front and in backside of the active region are formed in the same laser device. A number of reflection peaks are generated in the DBR regions in front and in backside of the active region by the uneven pattern of diffraction grating, and intervals of reflection peaks in the front DBR region is slightly different from intervals of reflection peaks in the backside DBR regions. “Vernier effect” can be obtained by this structure, which enables to tune a wavelength over a very wide range. The DBR-LD using the uneven pattern of diffraction grating can operate to tune over a wavelength range of 100 nm and more and to tune quasi-continuously a wavelength range of 40 nm.
On the other hand, a wavelength tunable laser proposed as the latter type which returns a specific wavelength into a laser device by turning over a diffraction grating. This type of wavelength tunable laser requires monitoring an oscillating wavelength gradually. Conventionally, a wavelength selective component such as Etalon is installed in the module to monitor an oscillating wavelength.
[Non-Patent Literature 1] “Optical Integrated Devices” by Kohroh Kobayashi, pp 104-122, the second impression of the first edition, published by Kyoritsu Shuppan Co., Ltd., December 2000
Many types of configurations are proposed as the conventional wavelength tunable lasers, however, they have disadvantages such as a mode hopping occurrence, a complicated method for wavelength control, low vibration resistance and high price due to an enlarged device. Therefore, it has been difficult for those lasers to be put into practical use.
As for the DBR-LD, a wavelength can be tuned by injecting carriers into the DBR region so as to vary a refraction index of the wavelength at the DBR region. However, current injection causes a formation of a crystal defect, which changes a rate of refractive index fluctuation to current injection extremely. Accordingly, it is difficult to keep laser oscillation at a certain wavelength over a long term. Moreover, because a compound semiconductor cannot be processed in and bigger than 2-inch size with the current technology, a laser device becomes enlarged and complex, so the current cost is difficult to be reduced.
On the other hand, as for the configuration where a wavelength tunable mechanism is set outside of a laser device, mode jumping occurs easily due to oscillation, so a grand mechanism with vibration resistance is required to avoid it. Accordingly, the module size becomes enlarged and the price increases.